A telecine unit that converts a film material recorded on an optical film for a movie and so forth into a television signal has been proposed. Generally, on a film material that is used for a movie theater, pictures have been recorded at a frame rate of 24 Hz (24 frames per second). Thus, the frame rate of a film material is completely different from that of an NTSC format television signal at a frame rate of 29.97 Hz. Thus, in the telecine unit, a process for converting 24 frames into 30 frames is performed. In such a process, two fields of an original film material are converted into three fields in a predetermined sequence. Thus, such a process is referred to as 2:3 pull-down process. In reality, a particular field of an original film material is repeated in a predetermined sequence. The repeated field is inserted between fields of the original film material (hereinafter, the repeated field is referred to as repeat field). Thus, with the film material at a frame rate of 24 Hz, a television signal at a frame rate of 30 Hz is generated.
Video data converted into a television signal by the telecine unit is compression-encoded by a compression-encoding technique such as MPEG encoding method. The encoded video stream is recorded on a record medium or transmitted to a transmission medium. Before video data that has been processed with the 2:3 pull-down process is compression-encoded, the repeat fields are removed so as to improve the compression-encoding efficiency. This is because the repeat fields are redundant fields that have been added with the 2:3 pull-down process. Thus, even if these repeat fields are removed, the picture quality does not deteriorate. A process for removing redundant fields added with the 2:3 pull-down process is referred to as inverse 2:3 pull-down process.
To remove repeat fields in the inverse 2:3 pull-down process, the repeat fields should be detected. To detect repeat fields, a simple algorithm is used. In the algorithm, the luminance difference between two fields (first and second fields) is calculated. When the luminance difference is almost “0”, it is determined that the second field is a repeat field.
However, since video data that has been processed with the 2:3 pull-down process is data of which a material optically recorded on an optical film is converted into a television data, the video data contains noise due to a miss-alignment of the film or dust and stain thereof. Thus, when video data that has been processed with the 2:3 pull-down process is processed with the inverse 2:3 pull-down process by a conventional repeat field detecting algorithm, if noise contained in the video data is small, repeat fields can be accurately detected. However, if noise contained in the video data is very large, normal fields (=not repeat fields) may be incorrectly determined as repeat fields.
In a broadcasting station, a video program production company, and so forth, video data generated from film material is not transmitted without editing process as a television program. Instead, a television program is generated by inserting new video data such as commercial program in the video data generated from film material by video editing process. The new video data is not video data generated from a film material, but video data (with a frame frequency of 29.97 Hz) that has been shot by a video camera or the like. In other words, the edited video program is including both video data generated from a film material by the 2:3 pull-down process (the frame frequency of the original material is 24 Hz) and normal video data (the frame frequency of the original material is 29.97 Hz).
When the inverse 2:3 pull-down process is performed for the edited video program using the above-described repeat field detecting algorithm, as long as the above-described noise is not abnormally large, repeat fields are removed from the video data generated from the film material. However, when the inverse 2:3 pull-down process is performed using the repeat field detecting algorithm, normal fields may be detected as repeat fields. When new inserted video data is similar to a still picture rather than moving picture, the probability of which normal fields are incorrectly detected as repeat fields becomes high.
In other words, in the conventional inverse 2:3 pull-down process, normal fields may be incorrectly detected as repeat fields. Thus, in the conventional inverse 2:3 pull-down process, repeat fields cannot be accurately removed. When normal fields are determined as repeat fields, the normal fields are removed from video data that has been processed with the inverse 2:3 pull-down process. As a result, field drop-outing of the normal field may occur.
Unlike with a storage system that records a supplied source material to a storage medium, in a digital broadcasting system, a source material should be processed and transmitted to individual subscribers on real time basis. Moreover, in the digital broadcasting system, the field drop-outing should be avoided from taking place in video data. In other words, in the digital broadcasting system, video data free of unnatural motion should be transmitted as an essential condition. The requirement of video data free of unnatural motion is superior to the requirement of the transmission efficiency using the inverse 2:3 pull-down process.
Thus, in a conventional digital broadcasting system, to completely prevent the field drop-outing from taking place in transmission video data, the inverse 2:3 pull-down process has not been performed at all. Consequently, the compression efficiency deteriorates by around 25% in comparison with the case that repeat fields are fully removed.